1. Field of the Invention
The present invention relates to a toner.
2. Discussion of the Background
Image formation by electrophotography is typically performed including the following processes:    (1) A latent electrostatic image formed on an image bearing member, for example, photoreceptors, is developed with a developing agent including a toner to form a toner image on the image bearing member (developing process);    (2) the toner image is transferred on a receiving material, for example, receiving paper, optionally via an intermediate transfer medium (transfer process); and    (3) the toner image is fixed on the receiving material upon application of heat, pressure, solvent vapor or the like (fixing process).
Developing a latent electrostatic image is typified into a liquid development method using a liquid developing agent in which various kinds of pigments and/or dyes are dispersed in insulative organic liquid and a dry development method, for example, a cascade method, a magnetic brush method and a powder crowd method, using a dry developing agent (hereinafter referred to as toner) in which a coloring agent, for example, carbon black, is dispersed in a natural or synthetic resin. In recent years, a dry development method has been diffused.
As a prevalent fixing method for use in the dry development method, a heat roller system is widely used in terms of its energy efficiency. To save energy by fixing toner at a low temperature, the thermal energy given during fixing is currently decreasing. In the Demand-side Management (DSM) program announced in 1999 by International Energy Agency (IEA), there is a technology procurement project for the next generation photocopier. In the requisite, drastic progress on saving energy is required such that, for a photocopier of 30 or higher cpm, the time taken to be ready recovering from stand-by status is within 10 seconds and the power consumption is from 10 to 30 W, depending on the copying speed, during stand-by status. A method of improving the temperature response of toner is conceivable to address the requisite but is not sufficient to obtain a good result.
To clear such requisites and minimize the stand-by time, technology speaking, it seems inevitable to lower the fixing temperature of toner so that the toner fixing temperature during operation is lowered.
To respond to the movement for this low temperature fixing, polyester resins, which have a relatively good low temperature fixing property and a good heat resistance preservation property, have been tried instead of using typically used styrene-acryl resins. In addition, there are other technologies, for example, published unexamined Japanese patent application No. (hereinafter referred to as JOP) S62-63940 describing adding a specific non-olefin crystalline polymer to a binder to improve the low temperature fixing property, and Japanese patent No. (hereinafter referred to as JP) 2931899 describing using crystalline polyester. However, it is difficult to say that the molecular structure and the molecular weight are not optimized therein.
Furthermore, it is impossible to achieve the goal of the DSM program by an application of these known technologies. Thus, it is desired to establish a relatively advanced technology for the improvement on the lower temperature fixing property of toner in comparison with the known technologies.
To further improve the lower temperature fixing property, there is a way of controlling the thermal characteristics of a resin. However, a glass transition temperature (Tg) of a resin that is excessively low causes deterioration of heat resistance preservation property thereof. Also a small molecular weight of a resin that invites too low an F1/2 temperature causes problems such that hot offset occurs at a low temperature. Resultantly, a toner having a good combination in terms of a low temperature fixing and a hot offset temperature has not been obtained by controlling the thermal characteristics of a resin.
With regard to methods of manufacturing a toner for use in developing latent electrostatic image, these methods are classified into pulverization methods and polymerization methods. In pulverization methods, toner is manufactured by uniformly dispersing a colorant, a charge controlling agent, an offset preventing agent, etc., in a thermoplastic resin through fusion mixing followed by pulverization and classification of the resultant. A good product of toner can be obtained by such a pulverization method but selection of materials for the toner is limited. For example, the composition obtained through fusion melting is desired to be pulverized and classified by a device available with a reasonable cost. Considering this point, the obtained composition through fusion melting is desired to be sufficiently brittle. When such a brittle composition is pulverized, obtained particles tend to have a wide particle size distribution. To produce images having a good definition and a good gradation, it is desired to remove fine particle having, for example, a weight average particle diameter of, for example, 4 μm or less and coarse particles having, for example, a weight average particle diameter of, for example, 15 μm or more. This may result in excessively low yield of toner. Also, it is difficult to uniformly disperse agents, for example, a colorant and a charge controlling agent, in a thermoplastic resin. Such non-uniform dispersion has an adverse impact on the fluidity, developability, durability, image quality, etc. of the resultant toner.
To address these drawbacks of the pulverization method, suspension polymerization methods have been proposed and performed in recent years. Manufacturing toner for use in developing latent electrostatic image by a polymerization method is already known to public. Toner particles are obtained by, for example, a suspension polymerization method or an emulsification polymerization method described in, for example, JP 2634503.
However, in these manufacturing methods, it is impossible to manufacture toner from polyester resins, which has an advantage in terms of the low temperature fixing property. To solve this drawback, for example, JOP H09-34167 describes a technology in which a polyester resin toner is made to have a spherical form by using a solvent in an aqueous medium and JOP H11-149180 describes a technology of obtaining a toner through an isocyanate reaction. However, these technologies are not satisfactory in terms of the low temperature fixing property and the productivity of toner.